The present invention relates generally to epoxy coatings, and more particularly to the use of blocked amine compounds in making epoxy coatings.
Curable epoxy resin systems are known. Conventional two component epoxy resin systems involve mixing of the epoxy resin and hardener and subsequent application of such mixtures as coatings by various techniques. Once mixed, such systems have short pot lives and must be used within a few hours.
As a result, efforts have been made to develop what are called one component systems. In one component systems, the curing agent is mixed with the epoxy but is inactive. It can be activated at a later time. One type of one component system involves the use of elevated temperature to activate the cross-linking reaction. However, the use of high temperatures is undesirable in many applications.
Another type uses latent cure, blocked amine systems in an attempt to alleviate the problem of reduced shelf life. In blocked amine systems, the amine is reacted with a ketone or aldehyde to form a blocked amine which is mixed with the epoxy resin. The system is activated by the addition of water, typically in the form of ambient moisture. This reverses the blocking reaction, forming the amine and the ketone or aldehyde. The amine then reacts with the uncured epoxy resin, and the ketone or aldehyde either evaporates or co-reacts with the epoxy. In the absence of moisture, such blocked amines systems afford a slight improvement in storage stability over conventional two component systems. However, commercial ketimine-based epoxy resin systems still suffer from limited storage stability, typically having a pot life of less than 24 hours. (Shell, 1986, EPON Curing Agents).
Furthermore, many systems include volatile organic compounds (VOC) or hazardous air pollutants (HAP), which are regulated. Commercial ketimine-based epoxy resin systems have elevated levels of VOCs, generally in excess of 3.5 lbs/gal. Under the current VOC standard for many industrial and maintenance coating applications, the limit is 3.1 lbs/gal of VOCs. This limit is likely to be reduced in the future to less than 2.8 lbs/gal of VOCs.
The use of ketimines as curing agents for epoxy resins is described in R. T. Holm, xe2x80x9cKetimines as Latent Epoxy Curing Agents,xe2x80x9d J. of Paint Tech., Vol. 39, No. 509, June 1967, pp. 385-388. The VOC levels of these compounds is over 3.5 lbs/gal. The reported viscosity of the formulations containing the various ketimines ranged from about 3 to about 36 stokes after storage for 20 days at 25xc2x0 C. However, these formulations do not provide the long term stability desired for commercial products. The long term stability of the formulations can be evaluated using accelerated aging testing at 55xc2x0 C. Two weeks storage at 55xc2x0 C. is equivalent to a shelf life of about six months, while 30 days storage is equivalent to a shelf life of over 1 year. The shelf life at 55xc2x0 C. is estimated to be only about 12% of the value at 25xc2x0 C.
British Patent No. 960,236, which is incorporated herein by reference, attempts to improve the shelf life of ketimine-based single component epoxy coatings by using hydroxyl-containing imines as blocked curing agents. The imines are obtained by reacting one or more imines possessing at least one amino hydrogen and one or more compounds having at least one epoxy group. No shelf life or VOC level is reported for these formulations. U.S. Pat. No. 5,837,785, which is incorporated herein by reference, discloses the use of heterocyclic containing curing agents for use in single component epoxy resin compositions. The heterocycle-containing compound has a backbone chain selected from the group consisting of polyether, polyvinyl, polyester, polyamide, polycarbonate, and novalac chains and at least two heterocyclic groups of the following general formula as side chains: 
wherein R1 and R2 may be the same or different and each represents hydrogen, straight chain or branched C1 to C6 alkyl or alkenyl, or C6 to C8 aryl; or R1 and R2 taken together with the adjacent carbon atom, represents C5 to C7 cycloalkyl: R3 represents C1 to C10 alkylene. A shelf life of 6 months at 40xc2x0 C. is reported, but no VOC level is given. The viscosity of coating formulations is not disclosed, but it appears to be high for conventional coating applications.
Therefore, there is a need for a single component epoxy coating precursor having improved shelf life and a method for making such a precursor. There is also a need for a low VOC epoxy coating and for a method of making such a coating. There is also a need for a method of making a blocked amine which can be used in a single component epoxy coating precursor.
The present invention solves this need by providing a single component epoxy coating precursor and a method for making such a precursor, a low VOC epoxy coating and a method for making such a coating, and a method for making a blocked amine which is more stable than previously known ones.
The single component epoxy coating precursor includes an epoxy resin, a first solvent, and a blocked amine. The single component epoxy coating precursor has a viscosity after 30 days at a temperature of 55xc2x0 C. of less than 16 stokes. It can have a viscosity after 30 days at a temperature of 55xc2x0 C. of less than 13 stokes, or a viscosity after 30 days at a temperature of 55xc2x0 C. of less than 7 stokes.
The method for making a single component epoxy coating precursor includes drying an epoxy resin and a blocked amine, combining and mixing the epoxy resin, the blocked amine, and a first solvent to form the single component epoxy coating precursor, wherein the single component epoxy coating precursor has a viscosity after 30 days at a temperature of 55xc2x0 C. of less than 16 stokes. It can have a viscosity after 30 days at a temperature of 55xc2x0 C. of less than 13 stokes, or a viscosity after 30 days at a temperature of 55xc2x0 C. of less than 7 stokes. This level of viscosity stability at 55xc2x0 C. generally corresponds to over one year of shelf life at room temperature storage conditions.
A reactive diluent optionally can be added to the single component epoxy coating precursor. Reactive diluents include, but are not limited to, modified glycidyl ethers, acrylates, methacrylates, urethane acrylates and combinations thereof. A water scavenger optionally can be added to the single component epoxy coating precursor. Water scavengers include, but are not limited to, molecular sieves, monocyclic bifunctional oxazolidines and combinations thereof. Pigments may be optionally added to the single component epoxy coating precursor. Pigments include, but are not limited to, titanium dioxide, diarylide yellow, iron oxide, raw umber, burnt umber, phthalocyanine blue, cobalt blue, chinese blue, phthalocyanine green, toluidine red, quinacridone red, dicerylide orange, carbon black, furnale black, lampblack, leafing aluminum and non-leaving aluminum.
Other formulating aids such as wetting agents, flow and rheology modifiers, light stability additives, etc., known in the art can be also incorporated.
First solvents which are useful in the present invention include, but are not limited to acetone, p-chlorobenzotrifluoride, t-butyl acetate, methyl isobutyl ketone, methyl propyl ketone and combinations thereof.
Epoxy resins include, but are not limited to, aliphatic epoxy resins, cycloaliphatic epoxy resins, aromatic epoxy resins and combinations thereof.
The single component epoxy coating precursor can have a VOC level of less than about 3 lbs/gal, or a VOC level of less than about 2.8 lbs/gal.
The method of making a low VOC epoxy coating includes drying an epoxy resin and a blocked amine, combining and mixing the epoxy resin, the blocked amine, and a first solvent to form the single component epoxy coating precursor, the single component epoxy coating precursor having a VOC level of less than about 3 lbs/gal, and exposing the single component epoxy coating precursor to water, the single component epoxy coating precursor and water reacting to form the low VOC epoxy coating. The water can be present in any form desired, including, but not limited to liquid water, and moisture in the air. The VOC level can be less than about 2.8 lbs/gal. The single component epoxy coating precursor described above and the blocked amine described below can be used to make the low VOC epoxy coating.
The method for making the blocked amine includes mixing a solvent capable of forming an azeotrope with water, an amine, and an amine blocker selected from ketones and aldehydes in a reaction vessel to form a reaction mixture. Ambient moisture is removed from the reaction vessel. The amine and the amine blocker are reacted to form the blocked amine and water of reaction, and the water of reaction is removed from the reaction mixture while the amine and the amine blocker are reacted. The blocked amine is recovered while maintaining the absence of moisture.
The solvent capable of forming an azeotrope with water includes solvents which can form binary or ternary azeotropes with water. These include, but are not limited to, toluene, xylene and combinations thereof.
The amine can be a polyamine, and it includes but is not limited to, diethylenetriamine, m-xylylenediamine and combinations thereof.
The amine blocker is selected from ketones and aldehydes. The ketones and aldehydes may have a molecular weight in the range of about 30 to about 600. They may have between about 2 and 14 carbon atoms. Suitable ketones include, but are not limited to, methyl isobutyl ketone, methyl ethyl ketone, acetone, phorone, heptanedione, tetramethylheptanedione, adamantone, acetonyl acetone, methylpropylketone and combinations thereof. Suitable aldehydes include, but are not limited to benzaldehyde, salicylaldehyde and combinations thereof.
The yield of blocked amine can be greater than about 90% of the theoretical yield, or greater than about 95% of the theoretical yield, or greater than about 97% of the theoretical yield.
The low VOC epoxy coating includes a reaction product of a single component epoxy coating precursor and water, the single component epoxy coating precursor comprising an epoxy resin and a blocked amine, the single component epoxy coating precursor having a VOC level of less than about 3 lbs/gal. The low VOC epoxy coating can have a VOC level of less than about 2.8 lbs/gal.